In order to transmit and receive a message and a file in real time and safely, there is a known method in which a chat message of text data transmitted from a chat server is converted into a chat message image of image data, and the chat message image is transmitted to a chat terminal device.
In a conventional chat system, in order to reduce a risk of correcting or altering the content of a chat message and leaking of information by text data extraction, a chat message is displayed based on image data (see, for example, Patent Document 1).